The objective of this project is to identify, clone, and sequence cellular proteins that bind to and regulate transforming potential of c-erbB1 gene products. Avian c-erbB1 encodes a receptor tyrosine kinase homologous to members of th family of human c-erbB proto-oncogene products including epidermal growth factor receptor *EGFR), c-erbB2 (HER2/neu) and c-erB3. Deletion f the ligand-binding domain of the avian c-erbB1 gene by proviral insertional activation causes erythroleukemia in birds by a ligand- independent mechanism. This truncated gene product can also cause tumors in other avian tissues (fibrosarcomas and hemangiomas) when additional specific mutations are introduced into the carboxyl-terminal domain. Since specific carboxyl-terminal deletions are able to impart tissue-specific oncogenicity, we hypothesize that:1)there must be proteins unique to different tissues that interact with the carboxyl-terminus of the avian c- erbB1 gene product, and 2) these proteins requlate this receptor's transformation. It is also likely that these proteins will have human homologues that are important in malignancies associated with abnormal expression of c-erbB oncogene products (including gastrointestinal, breast and ovarian carcinomas). My long-term goal is to define the mechanisms that link receptor tyrosine kinase activation with transformation in human gastrointestinal malignancies. The avian c-erbB1 system of tissue-specific oncogenicity is particularly well suited for the achievement of this goal. The specific aims of this proposal are to identify, clone and sequence avian proteins that interact with the carboxyl-terminus of the avian c- erbB1 gene product by employing current and novel molecular and biochemical techniques that have been pivotal for the cloning of regulatory proteins and substrates that interact with EGFR. This will include production of bacterially expressed affinity reagents in the form of c-erbB1 carboxyl- terminal fusion proteins, and utilization of the recently developed Cloning of Receptor Targets ('CORT') technique. Following accomplishment of these aims during Phase I, structure-function studies of these proteins in Phase II will be directed toward: 1) determining their tissue-specific expression and performing complementation studies in the avian system, and 2) identifying human homologues of these proteins in normal and malignant gastrointestinal tissues, and investigating their potential role in the regulation of human tyrosine kinases, in particular, the c-erbB family of gene products.